Carbon is known to have five unique crystalline structures, including diamond, fullerene (O-D nano graphitic material), carbon nano-tube (1-D nano graphitic material), graphene (2-D nano graphitic material), and graphite (3-D graphitic material). These are five fundamentally different and distinct classes of materials.
The carbon nano-tube (CNT) refers to a tubular structure grown with a single wall or multi-wall, which can be conceptually obtained by rolling up a graphene sheet or several graphene sheets to form a concentric hollow structure. Carbon nano-tubes have a diameter on the order of a few nanometers to a few hundred nanometers. Its longitudinal, hollow structure imparts unique mechanical, electrical and chemical properties to the material.
A graphene plane in a graphite crystal is composed of carbon atoms occupying a two-dimensional hexagonal lattice. The constituent graphene planes can be extracted or isolated from a graphite crystal to form individual graphene sheets. An isolated, individual graphene sheet is commonly referred to as single-layer graphene. A stack of multiple graphene planes bonded through van der Waals forces in the thickness direction is commonly referred to as a multi-layer graphene, typically having 2-300 layers or graphene planes, but more typically 2-100 graphene planes. Single-layer graphene and multi-layer graphene sheets are collectively called “nano-scaled graphene platelets” (NGPs). Our research group pioneered the development of graphene materials and related production processes as early as 2002: (1) B. Z. Jang and W. C. Huang, “Nano-scaled Graphene Plates,” U.S. Pat. No. 7,071,258 (Jul. 4, 2006), application submitted in October 2012; (2) B. Z. Jang, et al. “Process for Producing Nano-scaled Graphene Plates,” U.S. patent application Ser. No. 10/858,814 (Jun. 3, 2004); and (3) B. Z. Jang, A. Zhamu, and J. Guo, “Process for Producing Nano-scaled Platelets and Nanocomposites,” U.S. patent application Ser. No. 11/509,424 (Aug. 25, 2006).
NGPs are typically obtained by intercalating natural graphite flakes with a strong acid and/or oxidizing agent to obtain a graphite intercalation compound (GIC) or graphite oxide, as illustrated in FIG. 1. This is most often accomplished by immersing natural graphite flakes in a mixture of sulfuric acid, nitric acid (an oxidizing agent), and another oxidizing agent (e.g. potassium permanganate or sodium chlorate). The resulting GIC is actually some type of graphite oxide particles. This GIC is then repeatedly washed and rinsed in water to remove excess acids, resulting in a graphite oxide suspension or dispersion which contains discrete and visually discernible graphite oxide particles dispersed in water. There are two processing routes to follow after this rinsing step:
Route 1 involves removing water from the suspension to obtain “expandable graphite,” which is essentially dried GIC or dried graphite oxide particles. Upon exposure of expandable graphite to a temperature in the range of 800-1,050° C. for approximately 30 seconds to 2 minutes, the GIC expands by a factor of 30-300 to form a “graphite worm,” which is a collection of exfoliated, but largely un-separated or interconnected graphite flakes. In Route 1A, these graphite worms (exfoliated graphite or “networks of interconnected/non-separated graphite flakes”) can be re-compressed to obtain flexible graphite sheets that typically have a thickness in the range of 0.125 mm (125 μm)-0.5 mm (500 μm).
In Route 1B, the exfoliated graphite is subjected to mechanical shearing (e.g. using an ultrasonicator, high-shear mixer, air jet mill, or ball mill) to form separated single-layer and multi-layer graphene sheets (collectively, NGPs), as disclosed in our U.S. application Ser. No. 10/858,814. Single-layer graphene can be as thin as 0.34 nm, while multi-layer graphene can have a thickness up to 100 nm. In the present application, the thickness of multi-layer NGPs is preferably less than 20 nm.
Route 2 entails ultrasonicating the graphite oxide suspension for the purpose of separating individual graphene oxide sheets from graphite oxide particles. This is based on the notion that the inter-graphene plane separation has been increased from 0.335 nm in natural graphite to 0.6-1.1 nm in highly oxidized graphite oxide, significantly weakening the van der Waals forces that hold neighboring planes together. Ultrasonic power can be sufficient to further separate graphene plane sheets to form separate, isolated, or discrete graphene oxide (GO) sheets having a typical oxygen content of 5-47% by weight (more typically 20-47%). These graphene oxide sheets can then be chemically or thermally reduced to obtain “reduced graphene oxides” (RGO) typically having an oxygen content of 1-5% by weight, more typically <2% by weight.
For the purpose of defining the claims of the instant application, NGPs include single-layer and multi-layer graphene or reduced graphene oxide with an oxygen content of 0-5% by weight, more typically and preferably 0-2% weight. Pristine graphene has essentially 0% oxygen. In contrast, graphene oxide (GO) sheets, without reduction, have a typical oxygen content of 5-47% by weight (more typically 5-40%). Optionally, but preferably, GO sheets are originally in a gel form and, during or after merging with one another or with graphene sheets (NGP), the oxygen content can be reduced to less than 5%.
The present invention is directed at addressing unique thermal issues associated with a display device, such as a plasma display panel (PDP), a liquid crystal display (LCD), a light emitting diode (LED), and the like. These issues cannot be effectively addressed by current heat spreader materials, such as flexible graphite and the so-called “high-orientation graphite.”
A plasma display panel is a display device which contains multiple discharge cells, and is constructed to display an image by applying a voltage across electrodes of discharge cells thereby causing the desired discharge cell to emit light. A panel unit, which is the main part of a plasma display panel, is fabricated by bonding two glass base plates together in such a manner as to sandwich multiple discharge cells between them.
In a plasma display panel, each of the discharge cells which are caused to emit light for image formation generates heat. Thus, each cell is a heat source, which increases the temperature of other internal components and also causes the temperature of the plasma display panel as a whole to rise. The heat generated in the discharge cells is transferred to the glass that forms the base plates. However, it is difficult to conduct heat in the directions parallel to the panel face due to the low thermal conductivity of the glass base plate material.
Additionally, the temperature of a discharge cell which has been activated for light emission rises significantly, but the temperature of a discharge cell which has not been activated does not rise as much. Consequently, the panel face temperature rises locally in the areas where an image is being generated. Further, a discharge cell activated in the white or lighter color spectra generate more heat than those activated in the black or darker color spectra. Hence, the temperature of the panel face is different locally, depending on the colors generated in creating the image. Unless proper measures are taken to reduce these localized temperature differentials, they can accelerate thermal deterioration of affected discharge cells.
Because the temperature difference between activated and non-activated discharge cells can be high, and the temperature difference between discharge cells generating white light and those generating darker colors also can be high, a thermal stress is induced in the panel unit, causing the plasma display panel to develop thermal stress-induced cracks.
Furthermore, when the voltage applied to the discharge cell electrodes is increased, the brightness of the discharge cells increases and the amount of heat generated in such cells also increases. Hence, those cells having large voltages for activation become more susceptible to thermal deterioration and tend to exacerbate the cracking problem of the panel unit of the plasma display panel.
LEDs present similar issues with respect to heat generation as do plasma display panels. Similar issues arise in display devices other than emissive display devices, such as LCDs and LEDs, where hot spots can reduce the effectiveness or life of the device.
The use of so-called “high orientation graphite film” as thermal interface materials for plasma display panels to fill the space between the back of the panel and a heat sinking unit and to even out local temperature differences is suggested by Morita, et al in U.S. Pat. No. 5,831,374. The high orientation graphite used in the invention is highly crystalline graphite with graphite crystals oriented substantially in one direction. This graphite material is made by depositing overlapping layers of carbon atoms using a hydrocarbon-based source gas (also called CVD method), and then annealing the resulting structure. The high-orientation graphite may also be produced by carbonizing and then graphitizing a polymer compound at high temperatures (up to 3,200° C.). The precursor polymer compound may be selected from polyoxadiazoles (POD), polybenzothiazole (PBT), polybenzo-bis-thiazole (PBBT), polybenzooxazole (PBO), polybenzo-bis-oxazole (PBBO), polyimides (PI), polyamides (PA), polyphenylene-benzoimidazole (PBI), polyphenylene-benzo-bisimidazole (PPBI), polythiazole (PT), and polyparaphenylene-vinylene (PPV). This is a slow, tedious, energy-intensive, and high-cost process. Furthermore, Carbonization of certain polymers (e.g. polyacrylonitrile) involves the emission of toxic species.
In addition, U.S. Pat. No. 7,138,029, U.S. Pat. No. 7,150,914, U.S. Pat. No. 7,160,619, U.S. Pat. No. 7,276,273, U.S. Pat. No. 7,306,847, and U.S. Pat. No. 7,385,819 disclose the use of flexible graphite sheets (referred to as “sheets of compressed particles of exfoliated natural graphite” in these patents) as heat spreaders for a display panels. In these patents, the flexible graphite sheets are made by the compression of exfoliated graphite or graphite worms. The fabrication methods of exfoliated graphite have been well known and documented through numerous patents and technical publications. Briefly, the exfoliated graphite is re-compressed by using a calendering or roll-pressing technique to obtain flexible graphite sheets or foils, which are typically much thicker than 100 μm. It seems that no flexible graphite sheet thinner than 75 μm has ever been reported in the open literature or patent documents. Commercially available flexible graphite sheets normally have a thickness greater than 0.125 mm (125 μm), an in-plane electrical conductivity of 1-3×103 S/cm, through-plane (thickness-direction) electrical conductivity of 5-30 S/cm, in-plane thermal conductivity of 140-300 W/(mK), and through-plane thermal conductivity of approximately 5 W/(mK).
Flexible graphite sheets (obtained by re-compression of exfoliated graphite or graphite worms) for thermal management applications generally have the following major deficiencies:                (1) As indicated earlier, flexible graphite (FG) sheets exhibit a relatively low thermal conductivity, typically <500 W/mK and more typically <300 W/mK.        (2) Flexible graphite sheets are also of low strength and poor structural integrity. The high tendency for flexible graphite sheets to get torn apart makes them difficult to handle in the process of integrating them in a microelectronic device.        (3) Another very subtle, previously ignored or overlooked, but critically important feature of FG sheets is their high tendency to get flaky with graphite flakes easily coming off FG sheet surfaces and emitting out to other parts of a microelectronic device or display device. These highly electrically conducting flakes (typically 1-500 μm in lateral dimensions and >100 nm in thickness) can cause internal shorting and failure of electronic devices.        (4) In order to prevent the flaking of graphite particles, it becomes necessary to apply a protective coating onto a flexible graphite sheet, adding manufacturing complexity and cost to the heat spreader products and compromising the overall heat dissipation capability due to the extremely low thermal conductivity of the resin coating.        (5) The surface roughness of flexible graphite sheet is too large (about 500 nm) to form good contact with the display panel back. The real contact area is very small and therefore the thermal interface transfer efficiency is low.        
Similarly, the NGPs, when packed into a film or paper sheet of non-woven aggregates, exhibit a thermal conductivity higher than 1,000 W/mK only when the film or paper is cast and pressed into a sheet having a thickness lower than 10 μm. Further, the thermal conductivity is higher than 1,500 W/mK only when the film or paper is cast and pressed into a sheet having a thickness lower than 1 μm. This is reported in our earlier U.S. patent application Ser. No. 11/784,606 (Apr. 9, 2007). However, ultra-thin film or paper sheets (<10 μm) are difficult to produce in mass quantities, and difficult to handle when one tries to incorporate these thin films as a heat spreader material during the manufacturing process. Further, thickness dependence of thermal conductivity (not being able to achieve a high thermal conductivity at a wide range of film thicknesses) is not a desirable feature. Non-woven aggregates of NGPs (graphene sheets or platelets) also have a tendency to get flaky.
Our earlier application (U.S. application Ser. No. 11/784,606) further disclosed a mat, film, or paper of NGPs infiltrated with a metal, glass, ceramic, resin, and CVD graphite matrix material. Later on, Haddon, et al (US Pub. No. 2010/0140792, Jun. 10, 2010) also reported NGP thin film and NGP-polymer composites for thermal management applications. The processes used by Haddon et al to produce NGPs are identical to those disclosed much earlier by us (Jang, et al. U.S. patent application Ser. No. 10/858,814 (Jun. 3, 2004)). The NGP-polymer composites, as an intended thermal interface material, have very low thermal conductivity, typically <<2 W/mK, even though Haddon et al claim to have improved the thermal conductivity of epoxy by up to 3000%. The NGP films of Haddon, et al are essentially non-woven aggregates of graphene platelets, identical to those of our earlier invention (U.S. application Ser. No. 11/784,606). These aggregates have a great tendency to have graphite particles flaking from and separated from the film surface, creating internal shorting problem for the electronic device containing these aggregates. They also exhibit low thermal conductivity unless made into thin films (10 nm-300 nm, as reported by Haddon, et al) which are then very difficult to handle in a real device manufacturing environment. Balandin, et al (US Pub. No. 2010/0085713, Apr. 8, 2010) also disclosed a graphene layer produced by CVD deposition or diamond conversion for heat spreader application. More recently, Kim, et al (N. P. Kim and J. P. Huang, “Graphene Nanoplatelet Metal Matrix,” US Pub. No. 2011/0108978, May 10, 2011) reported metal matrix infiltrated NGPs. However, metal matrix material is too heavy and the resulting metal matrix composite does not exhibit a high thermal conductivity.
Another prior art material for thermal management application is the pyrolitic graphite film. The lower portion of FIG. 1 illustrates a process for producing prior art pyrolitic graphitic films or sheets. The process begins with carbonizing a polymer 46 at a carbonization temperature of 500-1,000° C. for 2-10 hours to obtain a carbonized material 48, which is followed by a graphitization treatment at 2,500-3,200° C. for 5-24 hours to form a graphitic film 50. This is a slow, tedious, and energy-intensive process. Carbonization of certain polymers (e.g. polyacrylonitrile) involves the emission of toxic species.
Thus, it is an object of the present invention to provide a highly thermally conductive composite thin film (composed of graphene oxide gel-bonded graphene sheets, also herein referred to as “GO gel-bonded NGPs” or “GO-bonded NGPs”) that can be used for thermal management applications (e.g. for use as a heat spreader) in a display device.
It is a particular object of the present invention to provide a highly conductive integrated graphene film (obtained by heat-treating graphene oxide gel to merge graphene oxide sheets in an edge-to-edge manner) and GO gel-bonded NGP composite thin-film structure that exhibits a thermal conductivity greater than 600 W/mK, typically greater than 800 W/mK, more typically greater than 1,500 W/mK (even when the film thickness is greater than 10 μm), and most preferably and often greater than 1,700 W/mK.
It is another object of the present invention to provide an integrated graphene film sheet that exhibits a relatively thickness-independent thermal conductivity.
Still another object of the present invention is to provide a GO-bonded pristine graphene composite thin film that exhibits exceptional thermal and electrical conductivity properties.
It is a further object of the present invention to provide an NGP-GO composite thin-film sheet that is lightweight and exhibits a relatively high strength or structural integrity.
It is yet another object of the present invention to provide a highly conductive NGP-GO composite thin-film sheet (and related processes) wherein the in-plane thermal conductivity is greater than 600 W/mK (preferably and typically greater than 1,000 W/mK) and in plane electrical conductivity is greater than 2,000 S/cm (preferably and typically >3,000 S/cm), and/or a tensile strength greater than 10 MPa (preferably and typically >40 MPa).